The disclosure relates generally to electric machine stators, and more particularly, to a retention assembly using a shim with a stator wedge of an electric machine and a related method.
Electric machines such as motors and generators include stator windings, also known as stator coils or bars, which are routinely inspected to verify their operation during scheduled outages. FIG. 1 is a perspective end view of a conventional electric machine 10. Electric machine 10 includes a stator core 16 having a plurality of stator slots 12 to accommodate a stator bar to generate an electro-magnetic flux. Stator slots 12 are configured to accommodate stator bars to be positioned in the stator slots defined around an inner circumference of stator core 16. The stator bars may be formed from a plurality of flat bar conductors that are coupled together to form a predetermined winding path. In one approach, the stator bars are fabricated from Roebel transposed rectangular copper strands. A rotor (not shown) may be disposed within an opening 18 in stator core 16 where an air or coolant gap is defined between the rotor and stator core 16. A partial, exploded view of the stator is illustrated by reference numeral 20 that is described in detail with reference to FIG. 2. Electric machine 10 may be any electrical rotating machine or dynamoelectric machine, including but not limited to a motor or generator.
FIG. 2 is a cross-sectional view of a conventional stator slot. Stator 40 may include a stator core 42 and is part of a dynamoelectric machine or electric machine, such as a motor or a generator. Stator core 42 includes a plurality of radially extending stator slots 44 for housing one or more stator bars; two stator bars 46 and 48 are shown. As will be appreciated, stator core 42 extends around a central axis and stator slots 44, as well as stator bars 46 and 48, extend longitudinally parallel to that axis and in a generally radially inward direction. In the illustrated form, side ripple springs 50 and 52 maintain stator bars 46, 48 firmly against the opposite sides of stator slot 44. Radial space in stator slots 44 may be taken up by radial fillers 60 and 70. A retention assembly 62 includes stator wedges 64 that extend longitudinally (into page) along a radially inner portion of stator slots 44 with their lateral edges residing in shaped mating grooves or dovetails 66 formed in stator slots 44. A top ripple spring 68 may be positioned at least partially within stator slot 44 such that top ripple spring 68 is adjacent to at least one slot filler 70. Top ripple spring 68 and stator bars 46, 48 may be secured in stator slot 44 using a plurality of stator wedge slides 72 (not always necessary) and stator wedges 64.
During operation, the stator bars of an electric machine are typically under multiple stresses such as electromagnetic and mechanical forces, chemical and thermal stresses. The mechanical stress imposed on a surface of stator bar(s) 46, 48 may be laterally, radially and axially applied. As shown in the detailed cross-sectional view of FIG. 3, over time, stator slot 44 contents can become loose due to various reasons such as but not limited to: material creep, material shrinkage, or wear on components such as ripple spring 68, stator wedges 64, dovetails 66, etc. When the contents becomes loose, it allows relative motion (arrow A) between the stator bar(s), slot contents such as top ripple spring 68, and stator core 42, which can cause stator bar vibration and abrasion. Excessive bar abrasion can be damaging to an electric machine and can necessitate an expensive stator bar rewind. To avoid damage to the electric machine, a typical maintenance item is to check stator wedge 64 tightness. If enough stator wedges 64 are determined to be loose, the unit is typically re-wedged. That is, the affected stator wedges are removed and new wedges inserted. The re-wedging process is time consuming and expensive because it requires hundreds of wedges to be cut out or driven out of position. For example, the re-wedging process can cost $100,000-300,000 and require 2-5 days of machine outage time. The re-wedging process can also cause damage to other components of the generator, adding costs. Several attempts have been made to avoid re-wedging over the years such as tightenable wedges incorporating integral radial jacking screws or the ability to further drive tapered wedge slides without disassembly. Most previous methods, however, required the replacement of the existing stator wedge. In addition, while re-wedging corrects the problem, the looseness may return after the new wedges are exposed to the same vibration and abrasion; thus, requiring another re-wedging process to be completed later.